The present invention relates to a continuous process for the production of aluminum chloride by the conversion of aluminum oxide bearing substances using reducing and chlorinating gases or gas mixtures in a fluidized bed containing a chemically and physically inert, solid dilution agent. The aluminum oxide bearing substances can be coated with carbon.
The method of producing aluminum chloride by the reduction and chlorination of aluminum oxide has been known for a long time and has increased in significance due to modern developments in the production of aluminum. There are several kinds of known chlorination processes of which the conversion of aluminum oxide bearing raw materials or pure alumina with a chlorinating and reducing gas mixture into the appropriate metal chloride is one of the best known. In that process pure alumina or aluminum oxide bearing raw materials is converted by means of chlorinating and reducing gases or mixtures of gases e.g. phosgene or chlorine/carbon monoxide, preferably at elevated temperatures (700.degree.-1000.degree. C.). EQU Al.sub.2 O.sub.3 +3CO+3Cl.sub.2 .revreaction.2AlCl.sub.3 +3CO.sub.2 ( 1) EQU Al.sub.2 O.sub.3 +3COCl.sub.2 .revreaction.2AlCL.sub.3 +3CO.sub.2 ( 2)
In an early work, W. D. TREADWELL and L. TEREBESI recognized that the rate of the reaction between aluminum oxide and a reducing and chlorinating gas mixture depends, besides the composition of the latter, also on the form of the aluminum oxide employed (W. D. TREADWELL and L. TEREBESI, Helv. Chim. Acta 15 (1932), 1352-1362, viz., 1355 ff. and 1362). These authors subjected the alumina to a pretreatment comprising heating at 950.degree.-1000.degree. C. and varying the duration between 2 and 10 hours. At the reaction temperature of 550.degree.-560.degree. C., which they found to be optimum, the conversion of the best active forms of aluminum oxide was found to be 88% after 30 minutes treatment with phosgene, and 62% after treatment with chlorine/carbon monoxide.
The conversion of aluminum oxide bearing raw material with chlorinating gases or gas mixtures and carbon as reactant has also been thoroughly investigated. In this case the endothermic reaction: EQU Al.sub.2 O.sub.3 +3C+3Cl.sub.2 .revreaction.2AlCl.sub.3 +3CO (3)
is to be considered as the main reaction which, in combination with the strongly exothermic secondary reaction given by equation (1), results in the likewise strongly exothermic overall reaction: EQU Al.sub.2 O.sub.3 +.sup.3 /2C+3Cl.sub.2 .revreaction.2AlCl.sub.3 +.sup.3 /2CO.sub.2 ( 4)
To carry through this process, the aluminum oxide bearing raw material such as pure alumina, clay, kaolin, bauxite etc. is coated or mixed with solid carbon and treated with a chlorinating gas e.g. sulphur chloride, phosgene, but in particular with chlorine and therefore chlorinated under reducing conditions. The carbon is in the form of pitch, tar, asphalt, bituminous coal or coke, whereby the porous products obtained by coking make the reaction with the gas containing chlorine. The reaction takes place below 1000.degree. C. and is exothermic. The aluminum oxide bearing raw material is coated with carbon either by mixing mechanically with powdered carbon and bricketting the mixture (TREADWELL and TEREBESI, supra), or by treating the raw material with hydrocarbons in the gaseous, liquid or solid form, whereby the hydrocarbon is cracked or coked (see details of literature under H. B. Muller, Reduction Chlorination of Pure Alumina and Bauxite, Thesis ETH Zurich, 1976, p. 6).
The reaction of aluminum oxide with carbon and chlorine begins at 375.degree.-385.degree. C. and also depends strongly on the kind of pre-treatment given to the aluminum oxide used (TREADWELL and TEREBESI, supra).
Whereas the older processes involved the treatment of uniform pieces of calcined aluminum oxide in shaft or rotary furnaces with reducing and chlorinating gas mixtures, the modern, large scale industrial processes, almost without exception, exploit the many technical advantages offered by carrying out the reaction in a fluidized bed. Of these advantages it is the possibility of carrying out the process continuously which stands out as the most significant. For example, fine particulate alumina was treated in a fluidized bed at 500.degree.-800.degree. C. with chlorinating and reducing gases, whereby CO and Cl.sub.2 were passed over a catalyst to form phosgene (German Pat. No. 948 972). In another case a fluidized bed process was combined with the use of an alkali-aluminum chloride melt (which acted catalytically) and the systematically optimized distribution of alumina particle sizes to achieve uniform operation of a three phase fluidized bed (German Pat. No. 1 061 757). In a further case using a fluidized bed process, the aluminum oxide bearing raw material was first brought into an active form with low residual moisture content and large specific internal surface area, and then converted with an appropriate gas or gas mixture (Swiss patent application No. 12713/74 by the same applicant).
Other known processes treat briquettes of a suitably pre-treated carbon coated aluminum oxide with the chlorinating gas or gas mixture in shaft or rotary furnaces, whereby the modern large scale processes, almost without exception, exploit the many technical advantages offered by carrying out the reaction in a fluidized bed. Of these advantages it is the possibility of carrying out the process continuously which stands out as the most significant. A mixture of aluminum oxide and low ash oil carbon (ash content below 2.5%) was therefore converted in a fluidized bed at 450.degree.-600.degree. C. However the reaction was guaranteed to take place smoothly without decomposition of the bed, only if the aluminum oxide and the oil carbon had similar particle sizes at the start of the reaction (German patent 1 237 995). In another process, in a first step at 750.degree.-1000.degree. C., a heavy oil was cracked on an aluminum oxide which had an internal surface area of at least 10 m.sup.2 /g, until the particles were coated with 10-14 wt.% of carbon. In a second stage the carbon coated aluminum oxide was chlorinated at 450.degree.-800.degree. C. (German patent 22 44 041). In another process alumina was dehydrated at 800.degree. C. to bring it into an active form having a specific surface area of 124 m.sup.2 /g and a water content of less than 1 wt.%, then coated with carbon at 800.degree. C. by cracking ethane, and finally chlorinated. For an 80% conversion of the alumina on the thermobalance a reaction time of 1.6 min was required at 800.degree. C., 9.1 min at 700.degree. C., and the start of the reaction was at 670.degree. C. (H. P. Alder/H. Baiker/H. Geisser/W. Richarz, The Chlorination of Alumina, a Comparison of the Kinetics with different Reduction Agents, 108th AIME Meeting, February 1979).
If chemical reactions between solid substances and gases are carried out in fluidized beds, where the fluidized bed of solid particulate material is one of the reaction partners and the flowing gaseous medium is the other, the reaction parameters depend essentially on heat and material transport in this fluidized bed. To achieve optimum yield from the reaction therefore one must strive for a high heat transfer coefficient and optimum mechanical mixing of material and gas in the bed. To this end, the formation of channels in the bed should in particular be avoided, and the diameter of the bubbles formed should be as small as possible.
The fact that in fluidized bed technology one frequently employs a relatively large spectrum of particle sizes and, in particular a large proportion of relatively fine solid particles (less than approximately 50 microns in diameter), aiming to satisfy both requirements, is counter productive. This undesirable fact can be overcome, and the exchange of heat and material in such fluidized beds improved to a significant degree, by adding a solid component of large particle size to a bed of small particle size.
The effect of this measure on heat transport in the fluidized bed has been proved experimentally many times. The effect of gas flow rate on heat transport in fluidized beds of aluminum oxide of 11 microns average particle size was therefore investigated after the addition of coarser particles in the form of glass balls of average diameter of 200 microns to the bed. It was found that with increasing flow rate of gas, without any addition, resulted in no increase at all in the heat transfer number h of the bed; on the other hand the addition of 30 and 47% of coarser particles resulted in a steep increase and a plateau region approximately at the level of twenty times the initial value (M. BEARNS: Proceedings of the International Symposium on Fluidization, June 6-9, 1967, Eindhoven, ed. A. A. H. Drinkenburg, Amsterdam 1967, p. 407 ff., in particular FIG. 3).
With respect to mixing in the fluidized bed, trials with fine zinc particles to which coarser particles of quartz or glass were added showed that, as a result of the addition of the coarser particles, both the radial and the axial mixing coefficients between gas and solid in a fluidized bed can be significantly improved (BEARNS et al., p. 408).
It has often been confirmed that the addition of material of different particle sizes prevents the formation of channels in fluidized beds, and reduces significantly the size of the bubbles which counter good mixing (DE GROOT, supra. p. 359, BEARNS, supra. pp. 393, 408, ZUIDERWEG, supra. p. 740).
Using this knowledge as a basis, theoretical considerations have been made about the optimum composition of fluidized beds made up of particles of different size. The results can be summarized as follows: in an optimum two component system the diameter of the fine particles should be 22.5% of that of the coarse particles, and the weight of the finer particles should make up altogether 25% of the total weight of the mixture (H. TRAWINSKI, Chem.-Ing. Techn. 23, 416 (1951), 25, 201, 229 (1953), F. A. ZENZ Petrol. Refiner, 32, 123 (1953), 36 4-11 (1957). It has often been confirmed experimentally that there is such an optimum for the ratio of coarse and fine particles in a fluidized bed mixture (e.g. DE GROOT, Proceedings, supra. p. 740).
If a solid reagent is transformed by a gas or gas mixture in a fluidized bed, then knowledge gained from the experiment is usually in the following form. Normally, in such gas-solid reactions the following limiting conditions have to be met:
(1) First, during the reaction, an excess of solid reagent over the gaseous reagent should be maintained so that the largest possible conversion and therefore a high utilization factor of the latter is assured. This depends on the kinetics and possibly further properties of the reaction in question to be carried out. PA1 (2) Secondly, the gas flow rate in the fluidized bed reactor should not exceed a value u.sub.t at which the solid reagent is carried i.e. blown out of the bed. Maintaining the flow rate is particularly difficult in those cases in which this reagent has a wide spectrum of grain sizes and masses, and as a result of the chemical conversion process where the mass and/or size of the individual particles become smaller and the spectrum of particle sizes becomes even larger. Usually a compromise is reached with respect to this limiting condition, and the small amounts of unreacted material which is possibly removed by the gas stream are removed from the waste gas and fed back into the reaction. PA1 (3) Finally, the rate should be chosen such that the minimum fluidizing rate u.sub.L for the heaviest particles is exceeded and the bed is prevented from breaking down into various layers of different particle masses. PA1 (a) On the one hand the gas flow rate should be much higher than the minimum fluidizing rate u.sub.L for the heaviest particles to prevent the bed from separating into layers. PA1 (b) On the other hand the gas flow rate should be much lower than the rate u.sub.t which results in the carrying out of the lightest particles to minimize the removal of these from the bed and therefore the reduction in yield which this would produce. PA1 (a) to make up the fluidized bed, a chemically and physically inert solid dilution agent is added to the aluminum oxide bearing substances in an amount equal to 10-90 wt.% of the total weight, and such that the aluminum oxide bearing reaction partner and the solid dilution agent have comparable average particle sizes and bulk densities at the start of the process, and PA1 (b) the solid dilution agent, as a result of the chemical conversion of the aluminum oxide bearing substance, becomes a component in the fluidized bed with a larger average particle size and weight than the reaction partner, and PA1 (c) during the reducing and chlorinating process, the flow rate of the gases in the fluidized bed reactor is set at a value of 20-100 times the minimum rate required to fluidize the aluminum oxide bearing reaction partner corresponding to a range between 2 and 30 cm/sec. PA1 (a) to form the fluidized bed, a chemically and physically inert solid dilution agent is added to the carbon coated aluminum oxide bearing substances in an amount equal to 10-90 wt.% of the total weight, and such that the carbon coated aluminum oxide bearing reaction partner and the solid dilution agent have comparable average particle sizes and bulk densities at the start of the process, and PA1 (b) during the chlorination process, the flow rate of the gases in the fluidized bed reactor is set at a value of 20-100 times the minimum rate required to fluidize the carbon coated reaction partner corresponding to a range between 2 and 30 cm/sec. PA1 (a) A gas mixture preferably of chlorine/carbon monoxide or phosgene is employed as the chlorinating/reducing gas or gas mixture for uncoated aluminum oxide particles. The reaction temperature was varied between 350.degree. C. and 800.degree. C., whereby a range of 400.degree.-600.degree. C. was found to be optimum. With chlorine/carbon monoxide the ratio of gases in the mixture can vary from 90 mole percent chlorine/10 mole percent carbon monoxide to 10 mole percent chlorine/90 mole percent carbon monoxide; a preferred gas mixture however is one in which the chlorine and the carbon monoxide are present in equi-molar concentrations. The gas mixture can be diluted with an inert carrier gas such as nitrogen, noble gases etc.
It is all the more difficult to meet this condition the larger the spectrum of particle sizes in the material in question, and can be made even more difficult if the mass of the individual solid particles changes (increases or decreases) as a result of the chemical reaction.
However, within these limiting conditions, which have to be determined in each individual case, an improvement in the exchange of heat and material in the fluidized bed allows an increase in gas flow rate and throughput of solids and therefore an increase in the performance of a fluidized bed reactor in terms of the volume of the unit and the time in operation. According to the knowledge available, these improvements in the thermal and mechanical properties of the fluidized bed can be achieved by adding to the bed a second solid material which, under the conditions in question, is inert and has different physical properties (particle mass, particle size, spectrum of particle size, bulk density).
This way the properties of a fluidized bed in which solid carbon is reacted with gases containing sulphur dioxide to reduce the latter to sulphur, are improved by adding an inert solid dilution agent to the bed. This dilution agent was selected on the basis of its wear resistance; apart from sand, alumina, magnesium oxide, alumino silicates, quartz or silicon carbide was employed. Under the given conditions the inert material had an average particle size between 150 and 1500 microns and the amount in the bed throughout the whole of the reaction was maintained at 90-99.9 wt.%.
Although the particle mass of the carbon fell continuously as a result of chemical conversion, this method allowed gas flow rates of over 30.5 cm/sec to be maintained. Also, the tendency of the carbon to agglomerate during the reaction could be effectively countered. At the same time it was noted that, as a result of the measures taken, the fluidized bed was more stable and had a more uniform temperature distribution (German Pat. No. 27 54 819).
For the same reason an inert material with a particle size of &gt;300 microns was added to a fluidized bed containing polymerides of ethylene or propylene in the form of a powder with a particle size of 1-300 microns. These were converted by gaseous chlorine to chlorinated poly-.alpha.-olefines. The preferred inert material added was polytetrafluoroethylene (PTFE) and this in a ratio of 0.1:1 to 10:1 to polyolefine powder. The reaction took place between 60.degree. and 145.degree. C. Although the mass of the individual particles of the reagents changed during the reaction as a result of chemical conversion, excellent throughput rates and high utilization of chlorine were achieved; without the inert additive the reaction could not be carried out even at 90.degree. C. as the granular polyolefine agglomerated (German Pat. No. 23 42 822).
Applying these results to the reduction of aluminum oxide by chlorination presents a number of difficulties which arise from the special physical conditions prevailing while carrying out this reaction:
Whereas it is relatively easy to achieve a high utilization factor for the gaseous reagent--as the basic chemical reaction takes place rapidly and completely at the temperatures over 500.degree. C. normally employed for this (H. P. MUELLER, Reduzierende Chlorierung von reiner Tonerde and Bauxit, Thesis, ETH Zurich, 1976, pp. 133 ff.)--it it much more difficult to satisfy the limiting conditions with respect to minimizing the amount of solid reagent which is removed from the bed by the emerging gas, as the physical properties of the solid reagent change in a complicated manner during the process as a result of the chemical reaction. It has been found that the average particle size of the reagent containing aluminum oxide does indeed remain the same until more than 80% conversion has taken place; however the interior of the particle is removed, and therefore the mass and the density of the individual particles decrease steadily (H. P. ALDER, H. P. MUELLER, W. RICHARZ, AIME Annual Meeting 1977, p. 219). If alumina produced by the Bayer process, having a particle range between 10 and 120 microns and an average particle size of 60 to 80 microns, is employed and fed continuously to the fluidized bed, then the bed contains both fresh unreacted particles 120 microns in diameter and an amount, which can vary to a greater or lesser degree, of strongly reacted particles, the size of which can be as small as 10 microns. If the rate of carry out u.sub.t of the individual particles is calculated as a function of particle density l.sub.s, one obtains: ##EQU1## where g is the force of gravity, l.sub.g the density of the gas in the fluidized bed, d.sub.p the diameter of the solid particle and .mu. is the viscosity of the gas (see D. KUNII/O. LEVENSPIEL, Fluidization Engineering, New York 1969, p. 76). From this it can be seen immediately that under the given conditions the flow rates for carry out of the largest and the smallest particles can differ by a factor of 2000 (0.03 cm/sec and 62 cm/sec).
Similar difficulties arise in connection with the minimum flow rate u.sub.L to achieve fluidizing conditions.
If this is calculated as follows: ##EQU2## where .phi. is a shape factor and .epsilon..sub.L is the volume between individual particles at the point of fluidization, it can be seen that, with the largest variation in particle size amounting to a factor of 12 (120 microns and 10 microns), the minimum flow rates for fluidization vary by a factor of about 1500. These conditions make the choice of a suitable gas flow rate extremely difficult for the following reasons:
In view of these difficult conditions it was an object of the present invention to increase as much as possible the specific space-time-yield of a given fluidized bed reactor in which the chlorination reduction of aluminum oxide is carried out. At the same time it was a further object of the invention to keep as high as possible the utilization factor of the gaseous reagent, expressed as a low concentration of unreacted gas. Furthermore, it is desired to diminish the amount of unreacted solid containing aluminum oxide which is carried out of the bed and to prevent the particles of different weight in the bed from segregating, and this during the full duration of the process. These and other objects of the invention will be obvious from the following description.